This invention relates to a twin roll continuous caster, and more particularly to a twin roll continuous caster that enables variation of the temperature of the casting rolls and the adjustment of the contour of the rolls during casting. A twin roll caster provides for continuous casting of thin metal strip from molten metal. The pair of casting rolls are laterally positioned to form a nip between the rolls and support a casting pool of molten metal on the casting rolls immediately above the nip. The molten metal may be poured from a ladle into a smaller vessel, or series of smaller vessels, from which it flows through a metal delivery nozzle located above the nip, forming the casting pool of molten metal extending the length of the nip. This casting pool is usually confined between side plates, or side dams, held in sliding engagement with the end surfaces of the casting rolls so as to restrain the two ends of the casting pool against overflow. The molten metal, supported on the counter-rotating casting rolls, is cooled on the casting surfaces of the casting rolls to form shells that are brought together at the nip between the casting rolls to form thin metal strip that is cast downwardly from the nip.
The twin roll continuous caster may be capable of continuously producing cast strip from molten steel through a sequence of ladles. Pouring the molten metal from the ladle into smaller vessels before flowing through the metal delivery nozzle enables the exchange of an empty ladle with a full ladle without interrupting the production of cast strip.
Specifically, the twin roll continuous caster cools the molten metal in the melt pool adjacent the casting surfaces of the casting rolls to form shells on the casting surface, which are brought together at the nip and continuously cast solidified thin strip downwardly from the nip between the casting rolls. To cool the casting surface of the casting rolls, cooling water is passed through the interiors of the casting rolls. Because the casting surfaces of the rolls are in contact with molten metal at, for example, a temperature of 1600° C., the temperature of the casting rolls are regulated to provide a desired temperature and heat flux from the molten metal in contact with the casting surfaces of the casting rolls. Typically, the casting rolls are maintained at a temperature of no more than approximately 400° C.
In casting thin strip by a twin roll continuous caster, the predictability of the crown in the casting surfaces of the casting rolls during a casting campaign is a difficulty. The crown of the casting surfaces of the casting rolls determines the thickness profile, i.e., the cross-sectional shape, of thin cast strip produced by the twin roll caster. Casting rolls with convex (i.e., positive crown) casting surfaces produce cast strip with negative (depressed) cross-sectional profile, and casting rolls with concave (i.e., negative crown) casting surfaces produce cast strip with a positive (i.e., raised) cross-sectional profile. The casting rolls are generally formed of copper or copper alloy with internal passages for circulation of cooling water and usually coated with chromium or nickel to form the casting surfaces. The casting rolls undergo substantial thermal deformation with exposure to the molten metal.
A problem exists where the contours of the casting rolls are altered axially and radially by the heat of the molten metal. The change in contour of the casting rolls, particularly radially, is manifested in the thickness profile of the thin cast strip that is cast. Hence, hitherto, the extent of deformation of the casting rolls during hot operation has had to be predicted prior to casting. Negative crowns are formed in the casting rolls, before casting while the rolls are cold, to provide a desired cast strip thickness profile taking account of the predicted extent of deformation of the casting rolls during the casting campaign to provide flat thin cast strip or thin cast strip with a slight crown, as desired. However, the crown shape of the casting surfaces during casting conditions is difficult to predict due to the changes in the temperature of the molten metal supplied to the casting pool, changes in the speed of casting, even slight changes in the composition of the metal and other variables. Therefore, the dimensions of the negative crown applied to the casting rolls when cold may not be appropriate during the entire casting campaign.
The variation in temperature and the temperature gradient across the casting rolls causes complex deformations in the cast strip in both the radial and axial directions. The thickness profile of the thin cast strip is shaped by the casting rolls, and especially by the variation of the contour of the edge portions of the strip. The variation in the thickness of the edge portions of the thin cast strip impacts the quality of the thin cast strip and also the hot rolling process performed after casting.
Also, the variation of the thickness profile of the thin cast strip may be the cause of functional difficulties with the pinch rolls designed to limit the deviation to left and right of the strip during rolling. Alternatively, or in addition, the variation of the thickness profile of the thin cast strip may be the cause of wrinkling and cracking of the strip after rolling.
The twin roll continuous casters of the prior art also contemplated alterations to the operating conditions during the casting campaign, e.g., by changing the casting rate or the volume of molten metal in the casting pool above the nip of the casting rolls. However, controlling and adjusting changes in the contours of the casting rolls during normal casting operation of the twin roll continuous caster was difficult at best. Any change in the casting rate or volume of molten metal in the casting pool results in a corresponding change in the strip profile. Furthermore, such changes in casting rate and volume of molten metal in the casting pool also change other casting parameters, such as strip gauge control, and therefore cannot readily be altered.
As described in Japanese Patent No. JP7-88599, the thermal deformation of the casting rolls of a twin roll continuous caster was found to depend on the temperature of the casting rolls, and therefore the contour of the casting rolls could be adjusted by altering the temperature of the casting rolls through external action. For example, a casting roll contour measuring instrument could be used to measure the extent of the casting roll crowns, or a cast strip profile measuring instrument could be employed to measure the cast strip crown. This data can be used to modify the output of a casting roll heating/cooling apparatus, and thereby regulate the surface contour of the casting rolls.
As described in Japanese Patent No. JP7-276004, the heat flux from the molten metal to the casting rolls could be varied and the cast strip crown and thickness could thereby be controlled. The device attempts to control the cast strip crown and thickness during casting by varying the heat flux from the molten metal to the casting rolls by employing sealing gas above the casting pool and adjusting the gas delivery temperature and/or the gas mixing proportions of the sealing gas. To achieve this temperature change, large amounts of sealing gas are needed to be delivered and adjusted. An elaborate apparatus was needed for that purpose since the sealing gas had low thermal conductivity making it difficult to alter the contour of the casting rolls evenly and consistently.
Moreover, in the prior twin roll casters, the cylindrical body of the casting roll typically included a main roll shaft that could be made to be watertight, and have pressurized tubes connected through the main roll shaft to the interior of a circumferential portion of the casting roll body. The circumferential portion of the cylindrical body was typically a watertight copper sleeve with internal cooling tubes in thermal engagement with the outer circumference of the circumferential portion, and preshaped to impart a concavely shaped reverse crown to the central portion of the casting roll to make possible the hydrostatic expansion and contraction of the circumferential body portion varying the contour of the casting roll through liquid pressure. Japanese Patent No. JP7-256401 discloses adjusting the liquid pressure delivered to the casting rolls in such a manner as to improve the extent of expansion (amount of reverse crown) of the casting rolls. However, an elaborate, large hydraulic device at high pressure was required in order to control the crown by the liquid pressure in this manner. Moreover, the contours of the casting rolls change greatly if the liquid pressure falls (for whatever reason) because the contours of the casting rolls are controlled by the liquid pressure.
The present invention provides a twin roll continuous caster that is able to alter the temperature distribution over the casting surfaces of the casting rolls and adjust the contour of the casting rolls during casting operations.
The twin roll continuous caster disclosed comprises a pair of casting rolls laterally positioned to form a nip there between, adapted to support a molten casting pool above the nip, and to produce a thin cast strip downwardly from the nip between the casting rolls. The casting rolls comprise a circumferential portion having a plurality of circumferential cooling passages, adapted to carry cooling liquid, adjacent a circumferential casting surface, and a temperature-regulating passage or passages, adapted to carry temperature-regulating liquid medium, spaced inward of the cooling passages.
The twin roll caster also comprises a cooling liquid circuit. The first cooling liquid circuit is adapted to circulate cooling liquid that has passed through the casting rolls from the outlets in the cooling passages to a cooler, such as a cooling tower, for cooling, and circulate the cooled cooling liquid from the cooler to the inlet in the first end portion of the cooling passages. A flow rate regulator is adapted to regulate the flow rate through the cooling liquid circuits.
The temperature-regulating passages are placed inwardly of the cooling liquid passages in the casting rolls. A temperature-regulating medium, typically water, from a supply unit is circulated to the temperature-regulating passages through inlets in said temperature-regulating passages and discharged through outlets in the temperature-regulating passages and circulated back to the supply unit. The temperature-regulating medium enables extensive deformation of the circumferential portion of the casting rolls, which in turn allows control of the contour of the casting rolls to be regulated during casting operations. The deformation of and contour of the casting surfaces of the casting rolls and in turn the profile of the cast strip is regulated by controlling the temperature and flow-rate of the temperature-regulating medium, which is supplied to the temperature-regulating passages of the casting rolls. This twin roll caster improves profile quality and yield of the thin cast strip that is formed with the casting rolls.
The temperature-regulating medium supply unit may further comprise of a temperature regulator to regulate the temperature of the temperature-regulating medium circulating through the temperature-regulating passages of the casting rolls.
Also, the twin roll continuous caster may further comprise a thermometer adapted to measure the temperature of the temperature-regulating medium to produce an output signal corresponding to the measured temperature of the temperature-regulating medium. Typically the thermometer is located at or near the outlets in the temperature-regulating passages. A profile detector may also be adapted to measure the profile of the thin cast strip, and produce an output signal corresponding to the measured profile of the thin cast strip. A controller may also be adapted to receive the output signal from the thermometer, the output signal from the profile detector and a target profile value of the thin cast strip, and regulate the temperature of the temperature regulating medium to produce thin cast strip of a desired profile. The profile detector may be replaced or enhanced by a contour detector adapted to measure the contour of the casting surface of the casting rolls, and produce a signal as an output corresponding to the measured contour value that is input to the controller. By use of a contour value input signal, the controller may be able to more accurately regulate the temperature of the temperature regulating medium by the temperature regulator.
The twin roll caster may also comprise a first temperature-regulating circuit of passages to circulate the temperature-regulating medium through the casting rolls and discharge the temperature-regulating medium at elevated temperature from the outlets in said temperature-regulating passages and circulate it to a cooler unit to cool the temperature-regulating medium. The temperature regulator measures temperature of the temperature-regulating medium circulated to the inlets of said temperature-regulating passages. A second temperature-regulating circuit of passages are also adapted to circulate temperature-regulating medium at elevated temperature discharged from the outlets in the temperature-regulating passages directly to said inlets of the temperature-regulating passages, and a volume flow rate regulator adapted to regulate the temperature and flow rate distributions of temperature-regulating medium circulated liquid between said first and second temperature-regulating passages.
Alternatively or in addition, the inner portion of the casting rolls may be adapted to provide one or more temperature-regulating passages to circulate the temperature-regulating medium. The inner portion has a first end portion and a second end portion. Through a first temperature-regulating circuit, the first end portion of the inner portion has an inlet adapted to receive a supply of temperature-regulating medium from the supply unit and a second end portion has an outlet adapted to discharge the temperature-regulating medium and circulate the same to a cooling unit of the supply unit. In addition, the inlet and outlet of the temperature-regulating passages may be connected through a second temperature-regulating circuit adapted to direct temperature-regulating medium from the outlet to the inlet. In any case, the temperature-regulating medium supply unit may include a temperature regulator adapted to regulate the temperature of the temperature-regulating medium. A flow-rate regulator is also provided for regulating the flow rate of the temperature-regulating medium through the first and second temperature-regulating circuits, as well as the flow rate distribution between the first and second temperature-regulating circuits.
The same or additional controller may be adapted to function with the inner portion of the casting rolls. In any case the controller may be adapted to regulate the flow rate of the temperature-regulating medium by a volume flow-rate regulator, and adapted to regulate the flow-rate distributions of the temperature-regulating medium flowing through the first and second temperature-regulating circuits by the same or a second volume flow-rate regulator. The controller may be further adapted to control the temperature of the temperature-regulating medium by a temperature regulator to regulate the temperature of the liquid medium flowing through the inner portion of the casting roll. One of ordinary skill in the art would appreciate and recognize that the second controller may be integrated into the same unit as the controller for regulating the temperature-regulating medium used in the outer circumferential portion of the casting rolls, if provided.